X-ray imaging can be performed with silver halide film to detect the x-rays directly or to detect electrons dislodged from an atom in the film by absorption of an x-ray. Each silver halide grain is too thin to absorb an appreciable fraction of the incident x-rays, so that silver halide film exhibits much less sensitivity to x-rays than to visible light. Increasing the film's thickness of silver increases its cost and degrades the spatial resolution of the images it can convey. A screen-film combination interposes a phosphor screen to convert each absorbed x-ray into a burst of visible photons, enhancing the response of the silver halide film. These phosphor screens are typically brittle slabs of refractive material, susceptible to breakage if dropped or bent. The mechanical inflexibility of the phosphor screen prevents conforming the screen-film combination to the patient, impairing comfort and image resolution.
X-ray images can also be acquired by an array of electronic detectors, either photodetectors sensitive directly to the x-rays or indirectly to visible light from interposed phosphors, or electron detectors sensitive to the electrons dislodged by x-ray absorption. Electronic detectors are active devices requiring external electrical power to register detection of x-rays by increasing the device's conduction of electrical current or change in voltage between two sensed locations along the electrical path. Some electronic detective arrays are fabricated as periodic structures on crystalline semiconductors. The crystalline semiconductive substrate must extend over an area wider than the largest object intended to be imaged, since only one-to-one imaging is typically performed because imaging lenses are not commercially practical for x-rays now. A crystalline or glass substrate extending more than several centimeters in two dimensions but thinner than a centimeter in its third dimension is expensive, structurally inflexible, and breakable. Each detective site in the array requires a continuous electrically conductive path to the array's image accumulator.
A radio frequency identification (RFID) tag including a photodetector is disclosed in U.S. Pat. No. 5,874,724 (Cato) for enabling a light-flash sequence to control which RFID tag responds to a predefined command from the RFID base station. A pulsed light source aimed toward this (RFID) tag-photodector combination “quickly and efficiently identifies individual items in a large group which is within a range of the base station,” according to Column 1, describes attaching a wavelength-selective filter to the photodetector but does not mention attaching material, such as phosphor, to increase the signal received by the photodetector. The claims are limited to a “directional signal” and associated detector, with one possibility being a light beam. U.S. Pat. No. 5,874,724 does not discuss using this combination of photodetector and RFID tag or arrays of this combination for imaging, nor integration of the photodetector with the RFID on a single substrate small enough to satisfy resolution constraints of imaging.
The present invention offers adequate x-ray sensitivity using a mechanically flexible, rugged substrate. The location of independent sensor-transponder combinations can be arranged as a periodic lattice, as a random arrangement, or as a collection of periodicities in order to ensure sampling of requisite spatial frequencies of the x-ray image in specific areas. The present invention does not require electrical connections spanning the distance between independent sensor-transponder combinations, allowing broader choices of the material occupying those spaces and greater latitude in their fabrication.